In one form of binary continuous inkjet (CIJ) printing, such as described in U.S. Pat. Nos. 6,554,410; 6,588,888; 6,863,385; and 6,866,370, a printhead produces fluid drops by thermal stimulation of the fluid jet of inkjet ink using ring heaters surrounding the nozzle orifice that initiate pinch-off of the fluid ligament and its induced reorganization into a spherical drop during flight. Unlike electrostatic deflection CIJ, the drops are not monodisperse, and two populations of relatively large drops and relatively smaller drops are intentionally produced. Typically, the large drops are employed as printing drops and the small drops are non-printing drops. Additional droplet types undesirably can form, including satellites and coalesced small non-printing drops. In one useful implementation, the volume of a large printing drop is threefold or fourfold that of a normal small non-printing drop; undesired merged non-printing drops (or catch drops) are 2× in volume. Printing and non-printing drops of ink are selected for marking the substrate and return to the ink tank, respectively, by means air deflection steering of the drop stream towards a catcher surface or gutter. Lateral direction of an air stream at the fluid droplet stream in flight imparts orthogonal momentum to the drops that succeeds in driving the non-printing drops to impact the gutter during a critical segment of the flight, but does not drive the printing drops quite so far, and their flight continues until they impact the substrate being printed. Understandably, the air flow must be carefully adjusted to accurately select between the drop populations, and apparatus for providing controlled gas flow is described in the above referenced patents and further, e.g., in U.S. Pat. Nos. 7,682,002 and 7,946,691. If the air flow is insufficient, such that small non-printing drops are not deflected far enough, they can reach the substrate being printed and a marking error of unintended printing occurs that is referred to as “dark defect” (DD), reducing print quality. If the air flow is too aggressive, large printing drops may also be swept into the gutter and not mark the substrate at all, creating another marking error due to the incomplete print image that is referred to as “pick out” (PO). The difference in the air flow settings (e.g., volumetric flow rate, or differential pressure) between the onsets of the two printing defects is referred to as “operating window,” “printing window,” “operating margin” or “printing margin”, or simply “print margin” or “print window.” It is typically desirable to enlarge the print window in order to maximize the robustness of the printing process. Operating settings of air flow through positive and negative ducts of the air deflection manifold are typically chosen by printing a test image, and then varying the air flow for each individual jetting module of a line head until it resides within the operating window between the onsets of the two defects. When the operating margin is properly established, dark defect should be practically eliminated.
Pigmented continuous inkjet inks are comprised of particles and the particle size distribution can profoundly affect the quality of drop formation. Increasing the average particle size of particulates dispersed in the ink increases the undesired merging of small, non-printing drops (or catch drops) that are 2× in volume, as described in U.S. Patent Application No. 2010/0321449 A1 to Clarke et al, which also leads to dark defect. The fresh ink original particle size distribution is accordingly chosen to provide good drop formation properties.
U.S. Pat. No. 7,163,283 B2 to Loyd et al. discloses that a fluid filter can be placed between the ink supply pump and the printhead of a continuous inkjet printer fluid system. Such in-line fluid filtration is typically designed to remove oversize particles not within the original particle size distribution of the fresh inks as supplied to the system (such as particles which may arise from adventitious contamination of the ink by ambient particles of dirt or debris, skin flakes, manufacturing residues, wear of the hardware components, and so forth, which contamination particles are typically of a size greater than 1 micrometer) so as to protect against nozzle plugging which may be caused by such oversize particles. The effective pore size of such in-line filter has accordingly been selected based on a fraction of the nozzle diameter, as filtration of particles above 1/10 of the effective diameter of the nozzle (or even above ⅓ or ½, as disclosed in U.S. Pat. Pub. No. 2011/0205319) is expected to be effective at preventing nozzle plugging, while minimizing pressure drop to achieve such desired particle filtration. As recent continuous inkjet printers have employed a nozzle size of approximately 9-10 micrometers, in-line filters have typically been selected to provide effective particle size filtration of approximately 1 micrometer. U.S. Pat. No. 7,163,283, e.g., discloses that the typical filter medium could be polymer-based (e.g., polypropylene) with a particle removal rating of 0.8 to 1.2 micrometers.
U.S. Pat. No. 4,460,904 to Oszakiewicz et al. and U.S. Pat. No. 4,658,268 to Needham, and published Japanese Application No. 03-023129 to Takatoshi are directed at electrostatic deflection continuous inkjet printing systems and have filtered fluid recirculation systems that are different from the ink supply loop to the printhead. In U.S. Pat. No. 4,460,904, the ink fluid recirculation system has a low-volume, high-pressure, filtered fluid path to the printhead, and the ink tanks is connected to a high-volume, low-pressure fluid path which also contains a filter; effective particle removal efficiencies are not disclosed. In U.S. Pat. No. 4,658,268, the primary ink tank filter of 3 micron pore size supplies ink through a 5-micron filter-damper to the printhead, and the return stream passes through a 20-micron gutter filter. In JP 03-023129, two fluid circulation pathways draw ink from the primary ink tank, one to supply the printhead and the other continuously stir the ink and redisperse settled pigment. Effective particle removal efficiencies are not disclosed, and pigment particles that settle are not of the colloidal domain and thus probably exceed 0.5 micrometers in effective particle diameter.